The present invention relates to a solids-stabilized water-in-oil emulsion used for enhanced crude oil recovery. More specifically, the stability of the solids-stabilized water-in-oil emulsion is enhanced by the method of pretreating the oil prior to emulsification. The pretreatment step can be accomplished by adding dilute acid to the oil, adding a lignosulfonate additive to the oil, sulfonating the oil, thermally treating the oil in an inert environment, thermally oxidizing the oil, and combinations thereof. The improved emulsion may be used either as a drive fluid to displace hydrocarbons from a subterranean formation or as a barrier fluid for diverting the flow of hydrocarbons in the formation.
It is well known that a significant percentage of oil remains in a subterranean formation after the costs of primary production rise to such an extent that further oil recovery is cost ineffective. Typically, only one-fifth to one-third of the original oil in place is recovered during primary production. At this point, a number of enhanced oil recovery (EOR) procedures can be used to further recover the oil in a cost-effective manner. These procedures are based on re-pressuring or maintaining oil pressure and/or mobility.
For example, waterflooding of a reservoir is a typical method used in the industry to increase the amount of oil recovered from a subterranean formation. Waterflooding involves simply injecting water into a reservoir, typically through an injection well. The water serves to displace the oil in the reservoir to a production well. However, when waterflooding is applied to displace viscous heavy oil from a formation, the process is inefficient because the oil mobility is much less than the water mobility. The water quickly channels through the formation to the producing well, bypassing most of the oil and leaving it unrecovered. For example, in Saskatchewan, Canada, primary production crude has been reported to be only about 2 to 8% of the original oil in place, with waterflooding yielding only another 2 to 5% of that oil in place. Consequently, there is a need to either make the water more viscous, or use another drive fluid that will not channel through the oil. Because of the large volumes of drive fluid needed, it must be inexpensive and stable under formation flow conditions. Oil displacement is most efficient when the mobility of the drive fluid is significantly less than the mobility of the oil, so the greatest need is for a method of generating a low-mobility drive fluid in a cost-effective manner.
Oil recovery can also be affected by extreme variations in rock permeability, such as when high-permeability xe2x80x9cthief zonesxe2x80x9d between injection wells and production wells allow most of the injected drive fluid to channel quickly to the production wells, leaving oil in other zones relatively unrecovered. A need exists for a low-cost fluid that can be injected into such thief zones (from either injection wells or production wells) to reduce fluid mobility, thus diverting pressure energy into displacing oil from adjacent lower-permeability zones.
In certain formations, oil recovery can be reduced by coning of either gas downward or water upward to the interval where oil is being produced. Therefore, a need exists for a low-cost injectant that can be used to establish a horizontal xe2x80x9cpadxe2x80x9d of low mobility fluid to serve as a vertical barrier between the oil producing zone and the zone where coning is originating. Such low mobility fluid would retard vertical coning of gas or water, thereby improving oil production.
For moderately viscous oilsxe2x80x94i.e., those having viscosities of approximately 20-100 centipoise (cP)xe2x80x94water-soluble polymers such as polyacrylamides or xanthan gum have been used to increase the viscosity of the water injected to displace oil from the formation. For example, polyacrylamide was added to water used to waterflood a 24 cP oil in the Sleepy Hollow Field, Nebr. Polyacrylamide was also used to viscosity water used to flood a 40 cP oil in the Chateaurenard Field, France. With this process, the polymer is dissolved in the water, increasing its viscosity.
While water-soluble polymers can be used to achieve a favorable mobility waterflood for low to moderately viscous oils, usually they cannot economically be applied to achieving a favorable mobility displacement of more viscous oilsxe2x80x94i.e., those having viscosities of approximately 100 cP or higher. These oils are so viscous that the amount of polymer needed to achieve a favorable mobility ratio would usually be uneconomic. Further, as known to those skilled in the art, polymer dissolved in water often is desorbed from the drive water onto surfaces of the formation rock, entrapping it and rendering it ineffective for viscosifying the water. This leads to loss of mobility control, poor oil recovery, and high polymer costs. For these reasons, use of polymer floods to recover oils having viscosities in excess of 100 cP is not usually technically or economically feasible. Also, performance of many polymers is adversely affected by levels of dissolved ions typically found in formations, placing limitations on their use and/or effectiveness.
Water and oil macroemulsions have been proposed as a method for producing viscous drive fluids that can maintain effective mobility control while displacing moderately viscous oils. For example, water-in-oil and oil-in-water macroemulsions have been evaluated as drive fluids to improve oil recovery of viscous oils. Such emulsions have been created by addition of sodium hydroxide to acidic crude oils from Canada and Venezuela. The emulsions were stabilized by soap films created by saponification of acidic hydrocarbon components in the crude oil by sodium hydroxide. These soap films reduced the oil/water interfacial tension, acting as surfactants to stabilize the water-in-oil emulsion. It is well known, therefore, that the stability of such emulsions substantially depends on the use of sodium hydroxide (i.e., caustic) for producing a soap film to reduce the oil/water interfacial tension.
Various studies on the use of caustic for producing such emulsions have demonstrated technical feasibility. However, the practical application of this process for recovering oil has been limited by the high cost of the caustic, likely adsorption of the soap films onto the formation rock leading to gradual breakdown of the emulsion, and the sensitivity of the emulsion viscosity to minor changes in water salinity and water content. For example, because most formations contain water with many dissolved solids, emulsions requiring fresh or distilled water often fail to achieve design potential because such low-salinity conditions are difficult to achieve and maintain within the actual formation. Ionic species can be dissolved from the rock and the injected fresh water can mix with higher-salinity resident water, causing breakdown of the low-tension stabilized emulsion.
Various methods have been used to selectively reduce the permeability of high-permeability xe2x80x9cthiefxe2x80x9d zones in a process generally referred to as xe2x80x9cprofile modification.xe2x80x9d Typical agents that have been injected into the reservoir to accomplish a reduction in permeability of contacted zones include polymer gels or cross-linked aldehydes. Polymer gels are formed by crosslinking polymers such as polyacrylamide, xanthan, vinyl polymers, or lignosulfonates. Such gels are injected into the formation where crosslinking reactions cause the gels to become relatively rigid, thus reducing permeability to flow through the treated zones.
In most applications of these processes, the region of the formation that is affected by the treatment is restricted to near the wellbore because of cost and the reaction time of the gelling agents. Once the treatments are in place, the gels are relatively immobile. This can be a disadvantage because the drive fluid (for instance, water in a waterflood) eventually finds a path around the immobile gel, reducing its effectiveness. Better performance should be expected if the profile modification agent could slowly move through the formation to plug off newly created thief zones, penetrating significant distances from injection or production wells.
McKay, in U.S. Pat. No. 5,350,014, discloses a method for producing heavy oil or bitumen from a formation undergoing thermal recovery. McKay describes a method for producing oil or bitumen in the form of oil-in-water emulsions by carefully maintaining the temperature profile of the swept zone above a minimum temperature, TcIf the temperature of the oil-in-water emulsion is maintained above this minimum temperature, the emulsion will be capable of flowing through the porous subterranean formation for collection at the production well. McKay describes another embodiment of his invention, in which an oil-in-water emulsion is inserted into a formation and maintained at a temperature below the minimum temperature. This relatively immobile emulsion is used to form a barrier for plugging water-depleted thief zones in formations being produced by thermal methods, including control of vertical coning of water. However, the method described by McKay requires careful control of temperature within the formation zone and, therefore, is useful only for thermal methods of recovery. Consequently, the method disclosed by McKay could not be used for non-thermal (referred to as xe2x80x9ccold flowxe2x80x9d) recovery of heavy oil.
A new process has recently been disclosed that uses novel solids-stabilized emulsions for enhanced oil recovery. U.S. Pat. No. 5,927,404 describes a method of using the novel solids-stabilized emulsion as a drive fluid to displace hydrocarbons for enhanced oil recovery. U.S. Pat. No. 5,855,243 claims a similar method of using a solids-stabilized emulsion, whose viscosity is reduced by the addition of a gas, as a drive fluid. U.S. Pat. No. 5,910,467 claims the novel solids-stabilized emulsion described in U.S. Pat. No. 5,855,243. U.S. Pat. No. 6,068,054 describes a method for using the novel solids-stabilized emulsion as a barrier for diverting the flow of fluids in the formation.
Preparing a solids-stabilized emulsion with optimum properties is key to successfully using the emulsion for enhanced oil recovery. Two important properties are an emulsion""s stability and its rheology. The solids stabilized emulsion should be shelf-stable, that is, the emulsion should be able to remain a stable emulsion without water or oil breakout when left undisturbed. In addition, the emulsion should be stable under flow conditions through porous media, i.e. in a subterranean formation. The emulsion""s rheological characteristics are also important. For instance, EOR methods for which this emulsion may be used include injecting the emulsion as a drive or barrier fluid into a subterranean formation. Accordingly, the emulsion should have an optimum viscosity for injection and to serve as either a drive or barrier fluid. In practicing EOR, and particularly with using the emulsion as a drive fluid, it is useful to match the rheology of the emulsion with the rheology of subterranean oil to be produced. Oil displacement using a drive fluid is typically more efficient when the drive fluid has a greater viscosity than that of the oil to be displaced. In addition to providing stability to the solids-stabilized emulsion, the invention described herein will allow the user to prepare solids-stabilized emulsions with a wide range of rheology to match that of the oil to be produced.
Because water and oil are readily available at most production sites, water-in-oil emulsions are a good choice for making the solids-stabilized emulsions for EOR. Some oils possess the chemical composition and physical properties necessary to make stable solids-stabilized water-in-oil emulsions with a wide range of solids. The added solids interact with components of oil, i.e., polars and asphaltenes, resulting in an increase in their effectiveness as surface-active agents. This interaction is specific to the type of solids and the composition of the oil to which they are added.
However, if the oil does not contain the right type and sufficient concentration of polar and asphaltene compounds, the addition of solids is ineffective because the solids are not adequately and suitably modified to function as stabilizers of the oil-water interface. Accordingly, some oils do not form stable solids-stabilized water-in-oil emulsions with any solids, or, some oils may form stable emulsions with some types of solids, e.g. silica, and may not form similar stable emulsions with other types of solids, e.g., clays and coal dust. The previously cited art suggests that asphaltenes or polar hydrocarbons may be added to these oils to improve their ability to form stable emulsions. U.S. Pat. No. 5,855,243, column 7, lines 6-10; U.S. Pat. No. 5,927,404 column 6, lines 44-47; U.S. Pat. No. 5,910,467 column 7, lines 3-6. However, this addition is not always successful because incompatibility between some oil components and the added asphaltenes and polars can result in phase separation or rejection of the added compounds. These cases limit the scope of the inventions disclosed in the U.S. Patents cited above.
To broaden the scope and improve the solids-stabilized emulsions described in U.S. Pat. Nos. 5,927,404, 5,855,243, 5,910,467, 6,068,054, an approach is needed that suitably modifies the oil composition so that it is responsive to the addition of solids for the preparation of stable water-in-oil emulsions. The present invention satisfies this need.
According to the invention, there is provided a method for enhancing the stability of a solids-stabilized water-in-oil emulsion, said method comprising the step of pretreating at least a portion of the oil prior to emulsification.
In one embodiment of the invention, the oil pretreatment step comprises the addition of dilute organic or mineral acid to at least a portion of the oil prior to emulsification.
In another embodiment of the invention, the oil pretreatment step comprises the addition of a lignosulfonate additive to at least a portion of the oil prior to emulsification.
In another embodiment of the invention, the oil pretreatment step comprises sulfonating at least a portion of the oil prior to emulsification.
In another embodiment of the invention, the oil pretreatment step comprises thermally treating at least a portion of the oil in an inert environment prior to emulsification.
In another embodiment of the invention, the oil pretreatment step comprises thermally oxidizing at least a portion of the oil prior to emulsification.
Combinations of these embodiments may also be used. Further disclosed is a method for producing hydrocarbons from a subterranean formation, comprising:
a) making a solids-stabilized water-in-oil emulsion with the pretreated oil;
b) contacting the formation with said solids-stabilized emulsion, and
c) producing hydrocarbons from the formation using said solids-stabilized emulsion.
Solids-stabilized water-in-oil emulsions have been generally described in U.S. Pat. Nos. 5,927,404, 5,855,243 and 5,910,467. Such emulsions are made by the process of combining oil with submicron to micron-sized solid particles and mixing with water until the solids-stabilized water-in-oil emulsion is formed.
As disclosed in the above referenced U.S. patents, the solid particles should have certain physical properties. The individual particle size should be sufficiently small to provide adequate surface area coverage of the internal droplet phase. If the emulsion is to be used in a porous subterranean formation, the average particle size should be smaller than the average diameter of pore throats in the porous subterranean formation. Methods for determining average particle size are discussed in the previously cited U.S. patents. The solid particles may be spherical in shape, or non-spherical in shape. If spherical in shape, the solid particles should preferably have an average size of about five microns or less in diameter, more preferably about two microns or less, even more preferably about one micron or less and most preferably, 100 nanometers or less. If the solid particles are non-spherical in shape, they should preferably have an average size of about 200 square microns total surface area, more preferably about twenty square microns or less, even more preferably about ten square microns or less and most preferably, one square micron or less. The solid particles must also remain undissolved in both the oil and water phase of the emulsion under the formation conditions.
The present invention allows the formation of stable solids-stabilized water-in-oil emulsions from oil that would otherwise lack adequate polar and asphaltene compounds to form such stable emulsions. The oil needed to make a stable emulsion using the method described by U.S. Pat. Nos. 5,927,404, 5,855,243 and 5,910,467, has to contain a sufficient amount of asphaltenes, polar hydrocarbons, or polar resins to stabilize the solid-particle-oil interaction. But, as noted, some oils do not have the sufficient type or amounts of these compounds to allow the formation of stable solids-stabilized emulsions. Pursuant to the present invention, the oil is pretreated to promote the formation of a stable solids-stabilized water-in-oil emulsion.
The oil used to make the solids-stabilized emulsion of the current invention can be oil of any type or composition, including but not limited to crude oil, refined oil, oil blends, chemically treated oils, or mixtures thereof. Crude oil is unrefined liquid petroleum. Refined oil is crude oil that has been purified in some manner, for example, the removal of sulfur. Crude oil is the preferred oil used to practice this invention, more preferably, the crude oil is produced from the formation where the emulsion is to be used. The produced crude oil may contain formation gas, or formation water or brine mixed with the oil. It is preferred to dehydrate the crude oil prior to treatment, however, mixtures of oil, formation gas and/or formation brine may also be used in this invention.
Preferably, formation water is used to make the emulsion, however, fresh water can also be used and the ion concentration adjusted as needed to help stabilize the emulsion under formation conditions.
Solids-stabilized water-in-oil emulsions according to the present invention are useful in a variety of enhanced oil recovery applications generally known in the art, including, without limitation, using such emulsions (a) as drive fluids to displace hydrocarbons in a subterranean formation; (b) to fill high permeability formation zones for xe2x80x9cprofile modificationxe2x80x9d applications to improve subsequent EOR performance; and (c) to form effective horizontal barriers, for instance, to form a barrier to vertical flow of water or gas to reduce coning of the water or gas to the oil producing zone of a well.
Attached in Table 1 are detailed physical and chemical property characterization data for three different types of crude oils which are referenced as Crude Oil #1, Crude Oil #2 and Crude Oil #3. Crude Oil #1 and Crude Oil #3 possess properties that enable formation of stable water in crude oil emulsions with the addition of solids, as described in U.S. Pat. Nos. 5,927,404, 5,855,243 and 5,910,467. However, Crude Oil #2 does not form a stable solids-stabilized water-in-oil emulsion when using the same method.
These differences suggest:
1. the surface-active species, i.e., asphaltenes and acids/resins, which are the key components essential for emulsification, are not readily available to stabilize the water droplets in Crude Oil #2, and
2. pretreatment of the oil to alter its physical properties and chemical composition is a potential route to enhance the stability of the emulsion.
Accordingly, the present invention describes a method of pretreating oil to increase the stability of the solids-stabilized emulsion. Several embodiments of this invention will now be described. As one of ordinary skill in the art can appreciate, an embodiment of this invention may be used in combination with one or more other embodiments of this invention, which may provide synergistic effects in stabilizing the solids-stabilized emulsion.
Pretreatment of Oil with Dilute Acid
One method of pretreating oil to enhance its ability to form a stable solids-stabilized water-in-oil emulsion is to pretreat the oil with dilute mineral or organic acid prior to emulsification. This acid pretreatment results in modifications to the oil and surface of the solids: (1) The basic nitrogen containing components of the oil are converted to the corresponding mineral or organic acid salts. These salts are more surface-active than the basic nitrogen containing components themselves and thus contribute to improving the stability of the solids-stabilized water-in-oil emulsion; (2) If the oil contains napthenic acids, the stronger mineral or organic acids displace the napthenic acids from the basic nitrogen containing compounds to which they are complexed thereby providing higher surface activity; (3) The protons from the acid act to protonate the anionic charged sites on the surface of the solids and thus modify the solids"" surface to improve its interaction with the surface-active components of oil (either preexisting in the oil or generated by the acid treatment); (4) If the oil contains calcium and naphthenic acids, the mineral or organic acids can displace the calcium and free the naphthenic acids, which are more surface-active than the calcium naphthenates.
Making the Solids-stabilized Water-in-Oil Emulsion Using Dilute Acid Pretreatment
To make this embodiment of the invention, dilute mineral or organic acid is added to the oil prior to emulsification. Solid particles can be added to the oil either before or after the acid pretreatment, but it is preferred to add the solids to the oil and then acid pretreat the oil with the solids. After the acid pretreatment and solids addition, the solids-stabilized emulsion is formed by adding water in small aliquots or continuously and mixing, preferably at a rate of between 1000 to 12000 rpm, for a time sufficient to disperse the water as small droplets in the continuous oil phase. It is preferred to have a water concentration in the water-in-oil emulsion of 40 to 80%, more preferably 50 to 65%, and most preferably 60%.
The acid is added to the oil with mixing, preferably for about 5 to 10 minutes at 25 to 40xc2x0 C. The preferred acid treat rate is between 8 and 30,000 ppm. The dilute acid may be mineral acid, organic acid, a mixture of mineral acids, a mixture of organic acids, or a mixture of mineral and organic acids. The preferred mineral acids are hydrochloric and sulfuric acid. However, other mineral acids can be used, including but not limited to perchloric acid, phosphoric acid and nitric acid. The preferred organic acid is acetic acid. However, other organic acids may also be used including, but not limited to para-toluene sulfonic, alkyl toluene sulfonic acids, mono di and trialkyl phosphoric acids, organic mono or di carboxylic acids (e.g. formic), C3 to C16 organic carboxylic acids, succinic acid, and petroleum naphthenic acid. Petroleum naphthenic acid can also be added to increase the surface-activity in the oil, or oils containing high naphthenic acid can be blended with the oils of interest to provide the increased surface-activity.
The solid particles are preferably hydrophobic in nature. A hydrophobic silica, sold under the trade name Aerosil(copyright) R 972 (product of DeGussa Corp.) has been found to be an effective solid particulate material for a number of different oils. Other hydrophobic (or oleophilic) solids can also be used, for example, divided and oil wetted bentonite clays, kaolinite clays, organophilic clays or carbonaceous asphaltenic solids. The preferred treat rate of solids is 0.05 to 0.25 wt % based upon the weight of oil.
After the emulsion is prepared, its pH can be adjusted by adding a calculated amount of weak aqueous base to the emulsion for a time sufficient to raise the pH to the desired level. It is desirable to adjust emulsion pH in the 5 to 7 range. However, adjusting pH is optional as in some cases it is desirable to inject an acidic emulsion and allow the reservoir formation to buffer the emulsion to the reservoir alkalinity.
Ammonium hydroxide is the preferred base for pH adjustment. Stronger bases like sodium hydroxide, potassium hydroxide and calcium oxide have a negative effect on emulsion stability. One possible explanation for this effect is that strong bases tend to invert the emulsion, i.e. convert the water-in-oil emulsion to an oil-in-water emulsion. Such an inversion is undesirable for the purposes of this invention.
In addition to increasing the stability of the solids-stabilized water-in-oil emulsion, the acid pretreatment method results in an emulsion with lower viscosity compared to one produced without acid pretreatment. This reduced viscosity aids in enhancing the injectivity of the emulsion. Thus, one may decrease the viscosity of a solids-stabilized emulsion by suitably adjusting the amount of acid pretreatment. This ability to manipulate the viscosity of the emulsion allows the user to optimally match the Theological characteristics of the emulsion to that of the oil to be recovered specifically for the particular type EOR method used. As noted in U.S. Pat. Nos. 5,855,243 and 5,910,467, gas may also be added to further lower the viscosity of the emulsion.
Another embodiment of this invention is to pretreat a slipstream or master batch of oil with dilute acid as described above and subsequently mix the slipstream with a main stream of oil prior to water addition and emulsification. This main stream of oil is preferably untreated crude oil, however, it may be any oil, including oil that has been treated to enhance its ability to form a stable emulsion or treated to optimize its rheology. If this slipstream method is used, the amounts of solids and dilute acid needed for the slipstream treatment are scaled accordingly to obtain the desired amounts in the resulting emulsion.